feat: add destroy command and session_id validation

- Add destroy subcommand for deleting sessions
- Add destroy --all for fresh start with confirmation
- Add -y flag to skip confirmation prompts
- Add validate_session_id() to reject empty session_ids
- Remove misleading force resume error message
- Update SKILL.md to v1.1 with destroy documentation

skills/kugetsu/SKILL.md

@@ -5,7 +5,7 @@ license: MIT
 compatibility: Requires opencode CLI, bash, and filesystem access for session state.
 metadata:
   author: shoko
-  version: "1.0"
+  version: "1.1"
 ---
 
 # kugetsu - OpenCode Session Manager
@@ -85,6 +85,23 @@ kugetsu stop mytask
 - Sends SIGTERM to process
 - Sets state to `idle`
 
+### kugetsu destroy `<session_id>` [-y]
+Delete a session:
+```bash
+kugetsu destroy mytask          # Prompts for confirmation (default: N)
+kugetsu destroy mytask -y        # Skips confirmation
+```
+- Errors if session is `used` (use `stop` first)
+- Errors if session not found
+
+### kugetsu destroy --all [-y]
+Delete all sessions:
+```bash
+kugetsu destroy --all            # Prompts for confirmation (default: N)
+kugetsu destroy --all -y         # Skips confirmation
+```
+- Useful for fresh start
+
 ### kugetsu help
 Show usage help.
 
@@ -94,6 +111,9 @@ Show usage help.
 start ──────────────► used ──────► idle (stop/SIGTERM)
                          │
                          └──────► left (kill/SIGINT/crash)
+                                        │
+                                        ▼
+                                    destroy (delete)
 ```
 
 ## Example Workflow
@@ -112,6 +132,9 @@ kugetsu resume issue42
 
 # Later, when done
 kugetsu stop issue42
+
+# When you want a fresh start
+kugetsu destroy --all
 ```
 
 ## Without kugetsu